1. Field of the Invention
The present invention relates to a method and apparatus for detecting the edge positions of a pattern and more particularly to a pattern edge position detecting method and apparatus designed so that a pattern including falls and formed on an object is scanned optically to detect the magnitude, etc., of diffused energy radiated from the fall edges of the pattern and thereby to obtain pattern information indicating for example the positions of the pattern edges, the distance between the pattern falls and the shape and dimensional features of the pattern with a high degree of accuracy. The method and apparatus of this invention are well suited for use in applications such as micro-measuring method and apparatus for measuring the linewidth of micro miniature circuit patterns formed on a mask or semiconductor wafer in the manufacturing process of integrated circuits (ICs) or method and apparatus for detecting the positions of marks on a wafer for mask-wafer aligning purposes by an exposure system in the photolithographic process.
2. Description of the Prior Art
In the manufacturing process of high-density ICs, the detection of circuit patterns on a wafer is effected by means of photoelectric detection employing a so-called radiation beam such as laser beam, high-intensity illuminating light or electron beam. In the case of a method disclosed for example in U.S. Pat. No. 4,112,309 dated Sept. 5, 1978, the convergent laser beam incident to the surface of an object, e.g., a wafer or mask is modulated at a specific frequency and subjected to oscillatory scanning to photoelectrically detect the diffused light scattered from the pattern edge of the object surface, analogically demodulate the resulting electric signal at the modulating frequency, subject the demodulated signal to differentiation (synchronous detection) and detect the zero point of the demodulated and differentiated signal or the center of oscillation of the laser beam as the pattern edge position.
However, this method has practical limitations in that the center of oscillations due to the beam modulation becomes unstable due to variation with time, that the slow detection response of a low-pass filter for synchronous detection purposes makes it impossible to increase the speed of relative movement of the object and the beam with respect to the oscillatory scanning by the beam and that an increase in the oscillation frequency (modulation frequency) of the beam for increasing the speed of relative movement makes the oscillation center unstable with the resulting deterioration in the accuracy of measurements.